This invention is concerned with the production of beams of high energy photons and/or neutrons and a method to use the beams to actively interrogate and detect concealed nuclear materials, radiological materials and chemical explosives.
In today's world, credible threat scenarios are posed by weapons-usable special nuclear materials, radiological weapons and chemical explosives. The most potentially catastrophic terrorist threat involving radioactive materials is the possibility of a self-sustained fission chain reaction detonated in an urban area. This scenario is credible and is taken very seriously by the federal authorities. Such an event could result in a significant number of deaths and massive devastation. The resulting fallout, containing highly radioactive fission and activation products, would contaminate many square miles. Such a device need only contain several kilograms to a few tens of kilograms of a fissile isotope. It could be transported in a shipping container or a small truck, and would be difficult to detect because of the relatively small amount of external radiation that it would produce, especially if shielded, before detonation.
Other credible threat scenarios include the use of ordinary radiological materials as potential weapons. A noninclusive list of potential threats includes the use of so-called “alternate nuclear materials” or the generation of “dirty” bombs, which use a combination of conventional explosives and nuclear material. Dirty bombs can be detected by passive radiation monitors if they are not shielded. If they are shielded, active interrogation using γ-rays is needed to detect the thick shield material required to hide the highly radioactive material.
Another very serious threat is that posed by bulk amounts of chemical explosives including improvised explosive devices, which could be explosives left on trains, car and truck bombs and others. These chemical explosives can be used to spread terror in the population or they can be used to produce long lasting threats to commerce by destroying bridges, tunnels and other commercially vital choke points.
The need to be able to detect special nuclear materials, alternate nuclear materials (radiological shielded “dirty” bomb) and chemical explosives leads to many requirements that are not met by current technologies (e.g., simple, unambiguous, inexpensive, rapid, detection of nuclear material, detection of chemical explosives, “dirty” bombs etc.).
Until recently, modern efforts to uniquely and unambiguously detect explosives stemmed from two unrelated occurrences: One was the downing of Pan American Flight 903 in 1989 and the other was the continuing effort to find rapid and reliable methods to detect buried explosives or land mines. In regard to airline security, the emphasis was on detection of small amounts of explosive in checked and carry-on airline baggage. The requirements for such a system were to detect quantities as small as 1 kg of nitrogen-based explosive at a rate of about one bag every 6 seconds. In addition, the cost of ownership had to be commensurate with the risk and the cost to society as a whole. In regard to land mines, the method had to be mobile over undeveloped terrain, easy to operate, technically effective and low cost.
With the events of 9/11 heralding a new level of terrorism throughout the world, the civilian need to detect explosives and now, weapons-usable special nuclear material and “dirty bombs,” has increased by orders of magnitude. From a technology for detecting small amounts of explosive in luggage, the problem has changed to detection of “dirty bombs”, small nukes, innovated chemical explosives and bulk amounts of explosives in trucks and cars. The risk factor has increased dramatically and so has the quantity of material and type of threat.
Electron and ion accelerators are often used to generate radiation that can penetrate materials to detect what's inside. X-rays produced by radioisotope sources and electron accelerators are used in inspection systems to inspect luggage, cargo, trucks and other containers. Neutrons produced in nuclear reactions by ion accelerators and neutron emitting radioisotopes are used for the same purposes and for detection of special nuclear materials.
Typically, a beam of charged particles, electrons or ions, is produced, accelerated to high energy and directed at a first target to produce the desired photon or neutron beam. The penetrating photon or neutron radiation produced by the reaction of the primary beam on the target passes through the closed container where it is attenuated according to the density of the material or it produces a specific response characteristic of the type of material. For example, nitrogen-based explosives resonantly absorb and re-emit gamma rays with energy of 9.17 MeV. Special nuclear materials can be made to fission and emit secondary penetrating neutron and photon radiation that can be detected and made to send an alarm. In addition, detectors can collect information on attenuation that can be used to determine density or to image the contents of the container. Resonant absorption can also be used to identify the elemental content of other materials inside. An alarm signal can be generated or an image of the contents can be produced.
It is a general requirement of an inspection process that inspection times should be as short as possible. This implies that the quantity of radiation that passes through a unit of area of the container in a unit of time should be as high as possible. This, in turn, implies that the electron or ion current and current density from the accelerator that produces the penetrating radiation be as high as possible. In an interrogation system that uses ionizing radiation, it is also important that the overall dose to the interrogated system be as low as possible. This requirement is based on the possibility that the container may include human stowaways and it is unacceptable to cause them physical harm. In addition, there may be other unknown physical, medical or biological devices or systems inside the container that would be damaged if a high dose is employed to perform the interrogation.
The requirements for short inspection times and low dose means the return signal emitted by the specific threat (explosives, special nuclear materials or shielded dirty bombs) should be highly specific of the threat material and easily distinguished for the background radiation.
The characteristics of the interrogating beam determine the specificity of the response signal. The characteristics include the type of radiation, the energy spectrum, its physical dimension and fluence, the angular divergence and the time structure of the beam. All these characteristics play important roles in determining the throughput rate, signal to noise and other information required to unambiguously characterize the hidden material inside a closed container. These characteristics of the interrogating radiation source are determined by the design and capabilities of the accelerator system that produces the primary beam and the by the design of the target where the interrogating radiation originates.
Past efforts to produce high current negative ions, accelerate them in an electrostatic tandem accelerator and transport the ions to a target have suffered serious beam current and beam emittance limitations. The problems stem from:    1. Past limitation of negative ion sources to produce low emittance, high output current negative ion beams;    2. Destruction of weakly bound negative ions due to collisions with gas in the ion source and in the low energy beam transport system before they are accelerated to high energy;    3. Voltage instability due to charge buildup on the insulating structure of the high voltage acceleration tube; and/or    4. Run away vacuum in the accelerating structure due to loss of stripper gas from the stripper canal and buildup of neutrals due to charge exchange with the ion beam.
There is therefore a need in the art for an accelerator/target system and accompanying methods capable of actively interrogating closed containers to detect the presence of special nuclear materials, radiological weapons and/or chemical explosives.